Water Absorbent Storage Layers

ABSTRACT

The present invention relates to improved water-absorbing storage layers for use in hygiene articles, the water-absorbing storage layers being essentially free of cellulose fibers.

The present invention relates to improved water-absorbing storage layersfor use in hygiene articles, the water-absorbing storage layers beingessentially free of cellulose fibers.

The production of water-absorbing polymer particles and the use thereoffor producing hygiene articles is described, for example, in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, especially on pages 252 to 258. Thewater-absorbing polymer particles are also referred to assuperabsorbents.

The currently commercially available disposable diapers consisttypically of a liquid-pervious topsheet (A), a liquid-imperviousbacksheet (B), a water-absorbing storage layer (C) between layers (A)and (B), and an acquisition distribution layer (D) between layers (A)and (C).

The water-absorbing storage layer consists typically of a mixture ofwater-absorbing polymer particles and cellulose fibers, thewater-absorbing polymer particles being fixed by the cellulose matrix.

In the last few years, there has been a trend toward ever thinnerdisposable diapers. To produce ever thinner disposable diapers, theproportion of cellulose fibers in the water-absorbing storage layer hasbeen lowered ever further. A disadvantage here is that the cellulosematrix is made ever thinner as a result, and the mobility of thewater-absorbing polymer particles in the water-absorbing storage layerincreases.

Especially when water-absorbing storage layers essentially free ofcellulose fibers are desired, i.e. consist virtually exclusively ofwater-absorbing polymer particles, there is the risk that thewater-absorbing polymer particles will slip within the disposable diaperor even fall out of the disposable diaper completely.

To solve this problem, novel water-absorbing storage layers have beenproduced. For example, WO 97/17397 A1 describes a process for producingwater-absorbing foams. Use of such foams allows the use of cellulosefibers to be dispensed with entirely. Cellulose-free hygiene articlescan also be secured to suitable nonwoven backsheets by fixing ofwater-absorbing polymer particles by means of thermoplastic polymers,especially of hotmelt adhesives, provided that these thermoplasticpolymers are spun out to form fine fibers. Such products are described,for example, in US 2003/0181115, US 2004/0167486, US 2004/071363, US2005/097025, US 2007/156108, US 2008/0125735, EP 1 917 940 A1, EP 1 913912 A1, EP 1 913 913 A2, EP 1 913 914 A2, EP 1 911 425 A2, EP 1 911 426A2, EP 1 447 067 A1, EP 1 813 237 A2, EP 1 813 236 A2, EP 1 808 152 A2,EP 1 447 066 A1. The production processes are disclosed in WO2008/155722 A2, WO 2008/155702 A1, WO 2008/155711 A1, WO 2008/155710 A1,WO 2008/155701 A2, WO 2008/155699 A1. A disadvantage is the relativelycomplex production process, since the spinning of the adhesive fibers inthe presence of water-absorbing polymer particles is difficult and proneto faults.

In addition, extensible cellulose-free hygiene articles are known, andUS 2006/0004336, US 2007/0135785, and US 2005/0137085 discloseproduction thereof by simultaneous fiber spinning of suitablethermoplastic polymers and incorporation of water-absorbing polymerparticles. This process too is complex and prone to faults.

It was an object of the present invention to provide improvedwater-absorbing storage layers for hygiene articles, especiallydisposable diapers. For the improved water-absorbing storage layers, itshould be possible to use the customary water-absorbing polymerparticles. Moreover, the improved water-absorbing storage layers shouldbe essentially free of cellulose fibers, and the water-absorbing polymerparticles in the water-absorbing storage layer should neither slip norfall out either in the dry or moist state. In the context of thisapplication, “free of cellulose fibers” means that the cellulose contentin the inventive storage layer is preferably less than 30% by weight,preferentially less than 20% by weight, more preferably less than 10% byweight, most preferably less than 5% by weight. Ideally, no cellulose atall is present.

The object is achieved by water-absorbing storage layers consisting of anonwoven backsheet, water-absorbing polymer particles and aliquid-pervious topsheet, wherein the water-absorbing polymer particlesare fixed on the nonwoven backsheet.

In one embodiment of the present invention, the liquid-pervious topsheetis adhesive bonded to the nonwoven backsheet to form pockets. For thispurpose, customary adhesives can be used. However, it is also possiblethat the liquid-pervious topsheet and/or nonwoven backsheet is entirelyor partly composed of a thermoplastic polymer, and the liquid-pervioustopsheet is adhesive bonded to the nonwoven backsheet by partialmelting. Suitable nonwoven backsheets may consist of mixtures ofthermoplastic fibers (for example polyolefins, polyesters, polyamides)and non-thermoplastic fibers (for example cellulose).

The formation of pockets filled with water-absorbing polymer particlesimparts the form of a quilt to the water-absorbing storage layer. Thewater-absorbing polymer particles are prevented from slipping within thewater-absorbing storage layer by the pockets.

In a further preferred variant of this embodiment, the depressions arepartly filled with a liquid-conducting filler material and the pocketsare optionally also additionally covered thereby. Useful fillermaterials for this purpose include hydrophilic fibers alone (for examplecellulose, viscose or rayon) or in a mixture with other fibers (forexample propylene or cellulose acetate). The fibers may also be thosewhich consist of more than one component and which have a bi- ormultilamellar or hollow cross section. Such fibers typically conduct theliquid better than simple smooth fibers.

Advantageously, the depressions formed in the water-absorbing storagelayer by virtue of the adhesive bonding of the liquid-pervious topsheetto the nonwoven backsheet are filled with further water-absorbingpolymer particles and fixed to a further liquid-pervious topsheet.

FIGS. 1 a and 1 b show cross sections, and FIG. 1 c shows a longitudinalsection, of the inventive water-absorbing storage layers of the firstembodiment, the reference numerals having the following meanings:

1 nonwoven backsheet

2 liquid-pervious topsheet

3 water-absorbing polymer particles

4 adhesive bond

5 second liquid-pervious topsheet

6 additional adhesive bond

7 machine running direction.

In a second embodiment of the present invention, a nonwoven substratewith preferably hydrophilic fibers protruding upward is used. Thewater-absorbing polymer particles are fixed by the fibers between thenonwoven backsheet and the liquid-pervious topsheet. The liquid-pervioustopsheet is preferably adhesive bonded to the fibers of the nonwovenbacksheet. The fibers protruding upward may consist of all knownpolymers and mixtures thereof, but preference is given to polyolefins,polyesters, polyurethanes, cellulose and derivatives thereof,polyamides. The fibers may also be those which consist of more than onecomponent and which have a bi- or multilamellar or hollow cross section.

FIG. 2 shows a cross section of the inventive water-absorbing storagelayers of the second embodiment, the reference numerals having thefollowing meanings:

8 nonwoven backsheet

9 liquid-pervious topsheet

10 water-absorbing polymer particles

11 fibers directed upward.

In a third embodiment of the present invention, a soft matrix composedof a liquid-pervious material is applied to the nonwoven backsheet, andthe water-absorbing polymer particles are introduced into the chambersof the matrix. The chambers of the matrix are sealed with aliquid-pervious topsheet. The soft matrix is preferably adhesive bondedto the nonwoven backsheet and the liquid-pervious topsheet.

An advantage of this embodiment is that the matrix material can beselected such that it additionally promotes liquid distribution withinthe water-absorbing storage layer. Suitable for this purpose are pressedhydrophilic fibers (for example of cellulose, chemically precipitatedcellulose or crosslinked cellulose), or open-pore soft sponges. In thecase of sponges, hydrophilic types are preferred. The matrix materialshould have, in the expanded state (unpressed), continuous pores withdiameter preferably of 0.001 to 2.0 mm, preferably of 0.01 to 1.0 mm,more preferably of 0.03 to 0.5 mm, most preferably of 0.06 to 0.3 mm.

FIG. 3 a shows a top view, and FIG. 3 b shows cross sections, of theinventive water-absorbing storage layers of the third embodiment, thereference numerals having the following meanings:

12 nonwoven backsheet

13 liquid-pervious topsheet

14 water-absorbing polymer particles

15 liquid-pervious matrix.

In all embodiments, in a further particularly preferred variant, it isadditionally possible to use a water-soluble adhesive for dry fixing ofthe water-absorbing polymer particles. The adhesive is applied, forexample, to the nonwoven backsheet before the application of thewater-absorbing polymer particles. The application can be effected, forexample, in punctiform fashion, over the whole area, or preferably instrips in or transverse to or diagonally with respect to the machinerunning direction. The water-soluble adhesive may consist, for example,of polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, starchand starch derivatives, cellulose and cellulose derivatives, orpolyacrylic acid. Most preferably, the water-soluble adhesive comprisesat least one polyamine or consists thereof. Suitable polyamines arepolyvinylamines, polyethyleneimines, polyallylamines. Particularpreference is given to polyvinylamine. On contact with moisture, theamine is released from the adhesive and becomes attached to the swellinghydrogel, which additionally causes a particular gel layer stability inthe swollen state.

In preferred embodiments, a web of the nonwoven backsheet is moved inmachine direction, and strips or geometric patterns comprisingwater-absorbing polymer particles are applied thereto. In the secondembodiment of the present invention, a continuous surface may beobtained in this way. In all three embodiments, however, any desiredgeometric forms and patterns are conceivable, for example one which arearranged like cushions comprising water-absorbing polymer particles interms of area. The cushions or the heaps of water-absorbing polymerparticles applied may assume any desired shape in terms of area, forexample circles, ellipses, rectangles, squares, triangles (viewed fromabove). Particular preference is given to any desired polygons ormixtures of polygons with which the two-dimensional surface can becovered without gaps. Particular preference is also given to theapplication of one or more continuous strips in machine runningdirection, the strips running parallel to one another.

In the case of pockets, it is advantageous to fill them loosely, inorder that the water-absorbing polymer particles can swell in asubstantially unhindered manner. Optionally, however, an elasticnonwoven can also be used as a topsheet or as a backsheet. Suchnonwovens are commercially available.

In all embodiments, the nonwoven backsheet is fixed on a suitablemachine by means of reduced pressure such that water-absorbing polymerparticles to be laid on can then be laid on there by means of masks orsimilar means, such that these water-absorbing polymer particles areheld fixed from below by the existing suction during processing. It isthus equally possible to temporarily fix the other components.

A. Water-Absorbing Polymer Particles

The water-absorbing polymer particles are produced by polymerizing amonomer solution or suspension and are typically water-insoluble.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid and itaconicacid. Particularly preferred monomers are acrylic acid and methacrylicacid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids, such as styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities can have a considerable influence on the polymerization. Theraw materials used should therefore have a maximum purity. It istherefore often advantageous to specially purify the monomers a).Suitable purification processes are described, for example, in WO2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitablemonomer a) is, for example, acrylic acid purified according to WO2004/035514 A1 comprising 99.8460% by weight of acrylic acid, 0.0950% byweight of acetic acid, 0.0332% by weight of water, 0.0203% by weight ofpropionic acid, 0.0001% by weight of furfurals, 0.0001% by weight ofmaleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% byweight of hydroquinone monomethyl ether.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The monomers a) typically comprise polymerization inhibitors, preferablyhydroquinone monoethers, as storage stabilizers.

The monomer solution comprises preferably up to 250 ppm by weight,preferably at most 130 ppm by weight, more preferably at most 70 ppm byweight, preferably at least 10 ppm by weight, more preferably at least30 ppm by weight, especially around 50 ppm by weight, of hydroquinonemonoether, based in each case on the unneutralized monomer a). Forexample, the monomer solution can be prepared by using an ethylenicallyunsaturated monomer bearing acid groups with an appropriate content ofhydroquinone monoether.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether(MEHQ) and/or alpha-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groupssuitable for crosslinking. Such groups are, for example, ethylenicallyunsaturated groups which can be polymerized free-radically into thepolymer chain, and functional groups which can form covalent bonds withthe acid groups of the monomer a). In addition, polyvalent metal saltswhich can form coordinate bonds with at least two acid groups of themonomer a) are also suitable as crosslinkers b).

Crosslinkers b) are preferably compounds having at least twopolymerizable groups which can be polymerized free-radically into thepolymer network. Suitable crosslinkers b) are, for example, ethyleneglycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycoldiacrylate, allyl methacrylate, trimethylolpropane triacrylate,triallylamine, tetraallylammonium chloride, tetraallyloxyethane, asdescribed in EP 0 530 438 A1, di- and triacrylates, as described in EP 0547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450A1, mixed acrylates which, as well as acrylate groups, comprise furtherethylenically unsaturated groups, as described in DE 103 31 456 A1 andDE 103 55 401 A1, or crosslinker mixtures, as described, for example, inDE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/032962A2.

Preferred crosslinkers b) are pentaerythrityl triallyl ether,tetraalloxyethane, methylenebismethacrylamide, 15-tuply ethoxylatedtrimethylolpropane triacrylate, polyethylene glycol diacrylate,trimethylolpropane triacrylate and triallylamine.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol, especially thetriacrylate of 3-tuply ethoxylated glycerol.

The amount of crosslinker b) is preferably 0.05 to 1.5% by weight, morepreferably 0.1 to 1% by weight, most preferably 0.3 to 0.6% by weight,based in each case on monomer a). With rising crosslinker content, thecentrifuge retention capacity (CRC) falls and the absorption under apressure of 21.0 g/cm² passes through a maximum.

The initiators c) used may be all compounds which generate free radicalsunder the polymerization conditions, for example thermal initiators,redox initiators, photoinitiators. Suitable redox initiators are sodiumperoxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodiumperoxodisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite.Preference is given to using mixtures of thermal initiators and redoxinitiators, such as sodium peroxodisulfate/hydrogen peroxide/ascorbicacid. The reducing component used is, however, preferably a mixture ofthe sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium saltof 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixturesare obtainable as Bruggolite® FF6 and Bruggolite® FF7 (BruggemannChemicals; Heilbronn; Germany).

Ethylenically unsaturated monomers d) copolymerizable with theethylenically unsaturated monomers a) bearing acid groups are, forexample, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethylmethacrylate, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

The water-soluble polymers e) used may be polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, modified cellulose,such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycolsor polyacrylic acids, preferably starch, starch derivatives and modifiedcellulose.

Typically, an aqueous monomer solution is used. The water content of themonomer solution is preferably from 40 to 75% by weight, more preferablyfrom 45 to 70% by weight, most preferably from 50 to 65% by weight. Itis also possible to use monomer suspensions, i.e. monomer solutions withexcess monomer a), for example sodium acrylate. With rising watercontent, the energy requirement in the subsequent drying rises, and,with falling water content, the heat of polymerization can only beremoved inadequately.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. The monomer solution can therefore be freed ofdissolved oxygen before the polymerization by inertization, i.e. flowingan inert gas through, preferably nitrogen or carbon dioxide. The oxygencontent of the monomer solution is preferably lowered before thepolymerization to less than 1 ppm by weight, more preferably to lessthan 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

Suitable reactors are, for example, kneading reactors or belt reactors.In the kneader, the polymer gel formed in the polymerization of anaqueous monomer solution or suspension is comminuted continuously by,for example, contrarotatory stirrer shafts, as described in WO2001/038402 A1. Polymerization on a belt is described, for example, inDE 38 25 366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a beltreactor forms a polymer gel, which has to be comminuted in a furtherprocess step, for example in an extruder or kneader.

To improve the drying properties, the comminuted polymer gel obtained bymeans of a kneader can additionally be extruded.

However, it is also possible to dropletize an aqueous monomer solutionand to polymerize the droplets obtained in a heated carrier gas stream.This allows the process steps of polymerization and drying to becombined, as described in WO 2008/040715 A2 and WO 2008/052971 A1.

The acid groups of the resulting polymer gels have typically beenpartially neutralized. Neutralization is preferably carried out at themonomer stage. This is typically done by mixing in the neutralizingagent as an aqueous solution or preferably also as a solid. The degreeof neutralization is preferably from 25 to 95 mol %, more preferablyfrom 30 to 80 mol %, most preferably from 40 to 75 mol %, for which thecustomary neutralizing agents can be used, preferably alkali metalhydroxides, alkali metal oxides, alkali metal carbonates or alkali metalhydrogencarbonates and also mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Particularly preferredalkali metals are sodium and potassium, but very particular preferenceis given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate and also mixtures thereof.

However, it is also possible to carry out neutralization after thepolymerization, at the stage of the polymer gel formed in thepolymerization. It is also possible to neutralize up to 40 mol %,preferably 10 to 30 mol % and more preferably 15 to 25 mol % of the acidgroups before the polymerization by adding a portion of the neutralizingagent actually to the monomer solution and setting the desired finaldegree of neutralization only after the polymerization, at the polymergel stage. When the polymer gel is neutralized at least partly after thepolymerization, the polymer gel is preferably comminuted mechanically,for example by means of an extruder, in which case the neutralizingagent can be sprayed, sprinkled or poured on and then carefully mixedin. To this end, the gel mass obtained can be repeatedly extruded forhomogenization.

The polymer gel is then preferably dried with a belt drier until theresidual moisture content is preferably 0.5 to 15% by weight, morepreferably 1 to 10% by weight, most preferably 2 to 8% by weight, theresidual moisture content being determined by EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP230.2-05 “Moisture Content”. In the case of too high a residual moisturecontent, the dried polymer gel has too low a glass transitiontemperature T_(g) and can be processed further only with difficulty. Inthe case of too low a residual moisture content, the dried polymer gelis too brittle and, in the subsequent comminution steps, undesirablylarge amounts of polymer particles with an excessively low particle sizeare obtained (fines). The solids content of the gel before the drying ispreferably from 25 to 90% by weight, more preferably from 35 to 70% byweight, most preferably from 40 to 60% by weight. Optionally, it is,however, also possible to use a fluidized bed drier or a paddle drierfor the drying operation.

Thereafter, the dried polymer gel is ground and classified, and theapparatus used for grinding may typically be single- or multistage rollmills, preferably two- or three-stage roll mills, pin mills, hammermills or vibratory mills.

The mean particle size of the polymer particles removed as the productfraction is preferably at least 200 μm, more preferably from 250 to 600μm, very particularly from 300 to 500 μm. The mean particle size of theproduct fraction may be determined by means of EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP220.2-05 “Particle Size Distribution”, where the proportions by mass ofthe screen fractions are plotted in cumulated form and the mean particlesize is determined graphically. The mean particle size here is the valueof the mesh size which gives rise to a cumulative 50% by weight.

The proportion of particles with a particle size of at least 150 μm ispreferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

Polymer particles with too small a particle size lower the saline flowconductivity or gel bed permeability (SFC or GBP). The proportion ofexcessively small polymer particles (fines) should therefore be small.

Excessively small polymer particles are therefore typically removed andrecycled into the process. This is preferably done before, during orimmediately after the polymerization, i.e. before the drying of thepolymer gel. The excessively small polymer particles can be moistenedwith water and/or aqueous surfactant before or during the recycling.

It is also possible in later process steps to remove excessively smallpolymer particles, for example after the surface postcrosslinking oranother coating step. In this case, the excessively small polymerparticles recycled are surface postcrosslinked or coated in another way,for example with fumed silica.

When a kneading reactor is used for polymerization, the excessivelysmall polymer particles are preferably added during the last third ofthe polymerization.

When the excessively small polymer particles are added at a very earlystage, for example actually to the monomer solution, this lowers thecentrifuge retention capacity (CRC) of the resulting water-absorbingpolymer particles. However, this can be compensated, for example, byadjusting the amount of crosslinker b) used.

When the excessively small polymer particles are added at a very latestage, for example not until in an apparatus connected downstream of thepolymerization reactor, for example to an extruder, the excessivelysmall polymer particles can be incorporated into the resulting polymergel only with difficulty. Insufficiently incorporated, excessively smallpolymer particles are, however, detached again from the dried polymergel during the grinding, are therefore removed again in the course ofclassification and increase the amount of excessively small polymerparticles to be recycled.

The proportion of particles having a particle size of at most 850 μm ispreferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

The proportion of particles having a particle size of at most 600 μm ispreferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

Polymer particles with too great a particle size lower the swell rate.The proportion of excessively large polymer particles should thereforelikewise be small.

Excessively large polymer particles are therefore typically removed andrecycled into the grinding of the dried polymer gel.

To further improve the properties, the polymer particles can be surfacepostcrosslinked. Suitable surface postcrosslinkers are compounds whichcomprise groups which can form covalent bonds with at least twocarboxylate groups of the polymer particles. Suitable compounds are, forexample, polyfunctional amines, polyfunctional amido amines,polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described inDE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, orβ-hydroxyalkylamides, as described in DE 102 04 938 A1 and U.S. Pat. No.6,239,230.

Additionally described as suitable surface postcrosslinkers are cycliccarbonates in DE 40 20 780 C1, 2-oxazolidone and its derivatives, suchas 2-hydroxyethyl-2-oxazolidone in DE 198 07 502 A1, bis- andpoly-2-oxazolidinones in DE 198 07 992 01, 2-oxotetrahydro-1,3-oxazineand its derivatives in DE 198 54 573 A1, N-acyl-2-oxazolidones in DE 19854 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amide acetals inDE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2 andmorpholine-2,3-dione and its derivatives in WO 2003/031482 A1.

Preferred surface postcrosslinkers are ethylene carbonate, ethyleneglycol diglycidyl ether, reaction products of polyamides withepichlorohydrin, and mixtures of propylene glycol and 1,4-butanediol.

Very particularly preferred surface postcrosslinkers are2-hydroxyethyloxazolidin-2-one, oxazolidin-2-one and 1,3-propanediol.

In addition, it is also possible to use surface postcrosslinkers whichcomprise additional polymerizable ethylenically unsaturated groups, asdescribed in DE 37 13 601 A1.

The amount of surface postcrosslinkers is preferably 0.001 to 2% byweight, more preferably 0.02 to 1% by weight, most preferably 0.05 to0.2% by weight, based in each case on the polymer particles.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to the surfacepostcrosslinkers before, during or after the surface postcrosslinking.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium, iron and strontium, trivalent cations such as thecations of aluminum, iron, chromium, rare earths and manganese,tetravalent cations such as the cations of titanium and zirconium.Possible counterions are chloride, bromide, sulfate, hydrogensulfate,carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate,dihydrogenphosphate and carboxylate, such as acetate, tartate, citrateand lactate. Aluminum sulfate, basic aluminum acetate and aluminumlactate are preferred. Apart from metal salts, it is also possible touse polyamines as polyvalent cations.

The amount of polyvalent cation used is, for example, 0.001 to 1.5% byweight, preferably 0.005 to 1% by weight, more preferably 0.02 to 0.8%by weight, based in each case on the polymer particles.

The surface postcrosslinking is typically performed in such a way that asolution of the surface postcrosslinker is sprayed onto the driedpolymer particles. After the spraying, the polymer particles coated withsurface postcrosslinker are dried thermally, and the surfacepostcrosslinking reaction can take place either before or during thedrying.

The spraying of a solution of the surface postcrosslinker is preferablyperformed in mixers with moving mixing tools, such as screw mixers, diskmixers and paddle mixers. Particular preference is given to horizontalmixers such as paddle mixers, very particular preference to verticalmixers. The distinction between horizontal mixers and vertical mixers ismade by the position of the mixing shaft, i.e. horizontal mixers have ahorizontally mounted mixing shaft and vertical mixers a verticallymounted mixing shaft. Suitable mixers are, for example, horizontalPflugschar® plowshare mixers (Gebr. Lödige Maschinenbau GmbH; Paderborn;Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BV;Doetinchem; the Netherlands), Processall Mixmill mixers (ProcessallIncorporated; Cincinnati; US) and Schugi Flexomix® (Hosokawa Micron BV;Doetinchem; the Netherlands). However, it is also possible to spray onthe surface postcrosslinker solution in a fluidized bed.

The surface postcrosslinkers are typically used in the form of anaqueous solution. The content of nonaqueous solvent and/or total amountof solvent can be used to adjust the penetration depth of the surfacepostcrosslinker into the polymer particles.

When exclusively water is used as the solvent, a surfactant isadvantageously added. This improves the wetting performance and reducesthe tendency to form lumps. However, preference is given to usingsolvent mixtures, for example isopropanol/water, 1,3-propanediol/waterand propylene glycol/water, where the mixing ratio is preferably from20:80 to 40:60.

The thermal drying is preferably carried out in contact driers, morepreferably paddle driers, most preferably disk driers. Suitable driersare, for example, Hosokawa Bepex® horizontal paddle driers (HosokawaMicron GmbH; Leingarten; Germany), Hosokawa Bepex® disk driers (HosokawaMicron GmbH; Leingarten; Germany) and Nara paddle driers (NARA MachineryEurope; Frechen; Germany). Moreover, it is also possible to usefluidized bed driers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. Equally suitable is a downstream drier, for examplea shelf drier, a rotary tube oven or a heatable screw. It isparticularly advantageous to mix and dry in a fluidized bed drier.Preferred drying temperatures are in the range from 100 to 250° C.,preferably 120 to 220° C., more preferably 130 to 210° C., mostpreferably 150 to 200° C. The preferred residence time at thistemperature in the reaction mixer or drier is preferably at least 10minutes, more preferably at least 20 minutes, most preferably at least30 minutes, and typically at most 60 minutes.

Subsequently, the surface postcrosslinked polymer particles can beclassified again, excessively small and/or excessively large polymerparticles being removed and recycled into the process.

To further improve the properties, the surface postcrosslinked polymerparticles can be coated or remoisturized.

The optional remoisturizing is carried out preferably at 30 to 80° C.,more preferably at 35 to 70° C. and most preferably at 40 to 60° C. Atexcessively low temperatures, the water-absorbing polymer particles tendto form lumps, and, at higher temperatures, water already evaporatesnoticeably. The amount of water used for remoisturizing is preferablyfrom 1 to 10% by weight, more preferably from 2 to 8% by weight and mostpreferably from 3 to 5% by weight. The remoisturizing increases themechanical stability of the polymer particles and reduces their tendencyto static charging.

Suitable coatings for improving the swell rate and the saline flowconductivity or gel bed permeability (SFC or GBP) are, for example,inorganic inert substances, such as water-insoluble metal salts, organicpolymers, cationic polymers and di- or polyvalent metal cations.Suitable coatings for dust binding are, for example, polyols. Suitablecoatings for counteracting the undesired caking tendency of the polymerparticles are, for example, fumed silica, such as Aerosil® 200, andsurfactants, such as Span® 20.

The water-absorbing polymer particles produced by the process accordingto the invention have a moisture content of preferably 0 to 15% byweight, more preferably 0.2 to 10% by weight, most preferably 0.5 to 8%by weight, the moisture content being determined by EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP230.2-05 “Moisture Content”.

The water-absorbing polymer particles produced by the process accordingto the invention have a centrifuge retention capacity (CRC) of typicallyat least 15 g/g, preferably at least 20 g/g, preferentially at least 22g/g, more preferably at least 24 g/g, most preferably at least 26 g/g.The centrifuge retention capacity (CRC) of the water-absorbing polymerparticles is typically less than 60 g/g. The centrifuge retentioncapacity (CRC) is determined by EDANA (European Disposables andNonwovens Association) recommended test method No. WSP 241.2-05“Centrifuge Retention Capacity”.

The water-absorbing polymer particles produced by the process accordingto the invention have an absorption under a pressure of 49.2 g/cm² oftypically at least 15 g/g, preferably at least 20 g/g, preferentially atleast 22 g/g, more preferably at least 24 g/g, most preferably at least26 g/g. The absorption under a pressure of 49.2 g/cm² of thewater-absorbing polymer particles is typically less than 35 g/g. Theabsorption under a pressure of 49.2 g/cm² is determined analogously toEDANA (European Disposables and Nonwovens Association) recommended testmethod No. WSP 242.2-05 “Absorption under Pressure”, except that apressure of 49.2 g/cm² is established instead of a pressure of 21.0g/cm².

B. Hygiene Articles

The hygiene articles, especially disposable diapers, consist of

-   -   (A) an upper liquid-pervious layer,    -   (B) a lower liquid-impervious layer,    -   (C) a water-absorbing storage layer (core) between layer (A) and        layer (B), and    -   (D) optionally an acquisition distribution layer between        layer (A) and layer (C).

The upper liquid-pervious layer (A) is the layer which has directcontact with the skin.

The material for this consists of customary synthetic or semisyntheticfibers, such as polyesters, polyolefins and rayon, or of customarynatural fibers, such as cotton. In the case of nonwoven materials, thefibers should generally be bonded by binders such as polyacrylates.Preferred materials are polyester, rayon, polyethylene andpolypropylene. Examples of liquid-pervious layers are described, forexample, in WO 99/57355 A1 and EP 1 023 883 A2.

The lower liquid-impervious layer (B) consists typically of apolyethylene or polypropylene film. However, it may also consist of anyother film-forming polymer, for example of polyester, polyamide,especially biodegradable polyester.

The inventive water-absorbing storage layers are essentially free ofcellulose fibers or have a proportion of cellulose fibers of preferablyless than 30% by weight, preferentially less than 20% by weight, morepreferably less than 10% by weight, most preferably less than 5% byweight. The water-absorbing polymer particles usable are not subject toany restriction. Preference is given, however, to using water-absorbingpolymer particles with a saline flow conductivity (SFC) of 50 to150×10⁻⁷ cm³ s/g, the saline flow conductivity (SFC) being determinableby the method described in WO 2008/092843 A1 (page 30, lines 16 to 36).

It is likewise possible to use water-absorbing polymer particles with agel bed permeability (GBP) of 10 to 100 darcies. In a particularembodiment, water-absorbing polymer particles with a gel bedpermeability (GBP) of 100 to 1000 darcies are used. The gel bedpermeability (GBP) is determined to US 2005/0256757.

It is additionally advantageous to use water-absorbing polymer particleswith a centrifuged retention capacity (CRC) of at least 33 g/g and anabsorption under pressure of 49.2 g/cm²(AUL0.7 psi) of at least 12 g/g.

It is additionally advantageous when the absorption rate of thewater-absorbing polymer particles for aqueous body fluids is adjustedoptimally to the particular demands in the water-absorbing storagelayer. To determine the absorption rate, preference is given to usingthe vortex test described in the literature, for example in themonograph “Modern Superabsorbent Polymer Technology”. F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, on pages 156 and 157. The vortex times ofthe water-absorbing polymer particles should be less than 120 seconds,preferably less than 80 seconds, preferentially less than 50 seconds,more preferably less than 40 seconds, most preferably less than 20seconds.

The acquisition distribution layer (D) consists typically of cellulosefibers, modified cellulose or synthetic fibers, and has the task ofrapidly absorbing aqueous liquids, for example urine, and passing themon to the water-absorbing layer (C).

For the acquisition distribution layer (D), preferably modified, morepreferably chemically modified, most preferably chemically stiffened,cellulose fibers are used. Suitable agents for chemical stiffening arecationically modified starches, polyamide-epichlorohydrin resins,polyacrylamides, urea-formaldehyde resins, melamine-formaldehyde resinsand polyethyleneimine resins.

The stiffening can also be effected by modifying the chemical structure,for example by crosslinking. The crosslinkers can crosslink the polymerchains by formation of covalent bonds. Suitable crosslinkers are, forexample, C₂- to C₈-dialdehydes, C₂- to C₈-monoaldehydes with acarboxylic acid group and C₂- to C₈-dicarboxylic acids.

According to the present invention, improved water-absorbing storagelayers are obtained, as are hygiene articles which comprise them. Theseparation of liquid storage and liquid conduction can firstlysignificantly lower material consumption, especially of fibers, forproduction of the storage layers; secondly, thin and soft hygienearticles are obtained, which have outstanding integrity when dry and inuse, since the water-absorbing polymer particles can be fixedsignificantly more efficiently.

1. A water-absorbing storage layer consisting of a nonwoven backsheet,water-absorbing polymer particles, and a liquid-pervious topsheet,wherein the water-absorbing polymer particles are fixed on the nonwovenbacksheet.
 2. The storage layer according to claim 1, wherein theliquid-pervious topsheet has been adhesive bonded to the nonwovenbacksheet to form pockets.
 3. The storage layer according to claim 2,wherein bridges between the pockets are filled with furtherwater-absorbing polymer particles and said further water-absorbingpolymer particles are fixed to a second liquid-pervious topsheet byadhesive bonding to the first liquid-pervious topsheet.
 4. The storagelayer according to claim 1, wherein the nonwoven backsheet has fibersdirected downward and the water-absorbing polymer particles are presentin the region of the fibers.
 5. The storage layer according to claim 4,wherein the liquid-pervious topsheet has been adhesive bonded to thefibers.
 6. The storage layer according to claim 1, wherein aliquid-pervious matrix is present between the nonwoven backsheet and theliquid-pervious topsheet.
 7. The storage layer according to claim 6,wherein the liquid-pervious matrix has been adhesive bonded to thenonwoven backsheet and the liquid-pervious topsheet.
 8. The storagelayer according to claim 1, wherein the water-absorbing polymerparticles have a centrifuge retention capacity of at least 15 g/g.
 9. Ahygiene article comprising a water-absorbing storage layer according toclaim 1.